This invention relates to deposition of thin film organic material on electronic displayes and more particularly to design of a deposition chamber which increases precision of final thickness and thickness uniformity of the thin film deposition.
Typically, a thin layer (thin film) of organic material is placed on electronic displays for different purposes. One such purpose is to provide a correct amount of brightness which is dependent on the thickness of the thin film. When a fixed voltage is applied to the thin film, depending on the thickness of the thin film, different brightness will be resulted. Therefore, for fabricating electronic displays with identical brightness, the thickness of different displays have to be identical and the voltage applied to each display has to be the same. The thickness and the uniformity of the thickness are not only essential for achieving the correct brightness but also are essential for other purposes of using a thin film.
To ensure good film quality, many process parameters have to be controlled. These include the cleanliness of the substrate, incidence angle of the evaporant, and heating the substrate to the required temperature for vapor attraction. However, the most critical factors of thin film production process are controlling the uniformity of layer thickness over the whole surface of the substrate and controlling the final thickness of the film.
Referring to FIG. 1, there is shown a prior art system of depositing a thin film of an organic material on an electronic display. Typically, the deposition process is performed in a vapor chamber 10. Vapor chamber 10 contains two vapor sources 12 and 12'. During the vapor deposition process, only one of the sources is active. For example, in FIG. 1, only vapor source 12 is active. Each vapor source 12 and 12' has a concave surface to hold the organic material 14 and 14', such as Tris (8-hyroxy) quinoline aluminum (Alq.sub.3), and an electrical circuitry to heat the organic material to a temperature in the range of 200-240 Celsius. Once the organic material 14 or 14' is heated, it starts vaporizing and creating a vapor stream 16 or 16' respectively. During the vapor deposition, vapor chamber 10 is sealed in order to reduce the pressure of the chamber by a vacuum (not shown). By reducing inside pressure of chamber 10, the vapor stream 16 or 16' raises in an even stream while creating a cosine distribution profile.
In operation, several electronic display substrates 20, such as glass silicon or ceramic substrates, are placed in the path of the vapor stream 16 or 16'. The curved distribution profile of the vapor streams 16 or 16' causes the vapor stream to deposit a non-uniform film on the display substrates 20.
The vapor chamber 10 has a crystal sensor 22 which is located at the same level and receives vapor deposit in the same manner as the display substrates 20. Sensor 22 sends out an output signal and the signal changes as the sensor receives more vapor deposit. The variation of the output signal of the sensor 22 during one run, from the start of a deposition process to the end of the process, indicates a representative value for the thickness of the vapor deposition on the display substrates during the same deposition process.
The output signal of the sensor 22 does not indicate the variation of the deposition thickness on different substrates and the variation of the deposition from one run to the next. In addition, the reason for having only one active vapor source during each deposition process is that sensor 22 is not capable of measuring variation between the vapor distributions of the two vapor sources 12 and 12'.
It is an object of this invention to provide an apparatus which is capable of substantially increasing the degree of uniformity of the thin film deposition and controlling the thickness of the thin film.